Narrowband Ultrasound Sensing
A prior art ultrasonic sensing system typically uses single-frequency (narrowband) transducer and time delay estimation to determine a distance to an object in a scene. Typically, the narrowband pulses have a center frequency around 40 kHz and bandwidth of 1-2 kHz; i.e., the pulses operate in the range of 40 kHz±1 kHz.
Specifically, the system measures a time delay between transmitting an ultrasonic pulse and receiving the first reflected pulse. The time delay is multiplied by the speed of the ultrasound pulse to an estimate of the distance traveled by a narrowband ultrasonic pulse. Later reflected pulses can provide additional information about the object. However, that information is difficult to discern, and current technology does not attempt to do so.
The further information can be deduced by an array of transducers, which transmit the same frequency pulse from each transducer with an appropriate time delay between pulses from different transducers, so that the overall transmission is essentially directed towards a particular position. This is sometimes called beam steering.
By scanning different directions, a receiver can estimate the distance of the nearest object in the particular direction. That approach is similar to methods used in medical ultrasound. That approach has the drawback that a large number of transducers is required to make an effective array. Furthermore, the scanning approach requires multiple pulses, which increases the time before the scene, i.e., the sensed objects, can be sufficiently reconstructed. Last, it is not possible to form virtual arrays by moving the array elements because all array elements need to be used simultaneously for beam steering.